Production of alloys of iron

ABSTRACT

The invention is concerned with the extraction of carbon from ferroalloys, iron and alloy steels by the use of a high-velocity high-temperature plasma jet of a gas having an affinity to form stable volatile carbon compounds but a passivity towards ferrous metals at such high temperatures. The jet is directed into a molten and superheated bath of the metal to enable the volatile carbon compounds to separate from the metal leaving the metal bath with an extra low carbon content. The plasma jet will preferably include water vapor.

United States Patent [72] Inventor William Bleloch [56] References Cited 17 K061011180 Road, Pll'ktOWll, UNITED STATES PATENTS 2,874,265 2/1959 Reed 75/10 [21] Appl. No. 788,788

3,257,196 6/1966 Foex..... 75/10 [22] Med Jan. 3, 1969 3,279,912 10/1966 Death... 75/10 [45 1 Paemd 1971 3 347 766 /1967 1) 111 75/10 [32] Priority Jam 10,1968 ea [33] South Africa Primary Examiner winston A. Douglas [31 68/0176 Assistant Examiner-Peter D. Rosenberg AltomeyShoemaker and Mattare [541 PRODUCTION OF ALLOYS OF IRON ABSTRACT: The invention is concerned with the extraction of carbon from ferroalloys, 1ron and alloy steels by the use of a Chums, 3 Drawmg Figs.

high-velocity high-temperature plasma et of a gas having an [52] US. Cl 75/12, affinity to form stable volatile carbon compounds but a pas- 75/1 1, 219/121 P sivity towards ferrous metals at such high temperatures. The [51] Int. Cl C2lc 5/52, jet is directed into a molten and superheated bath of the metal C2lc 5/52, B23b 9/00 to enable the volatile carbon compounds to separate from the Field of Search /10, 1 1, metal leaving the metal bath with an extra low carbon content.

The plasma jet will preferably include water vapor.

PATENTEDnm 26 I9?! 49 i INVENTOR 12 Y WILLIAM BLELOCH ATTORNEYS PRODUCTION OF ALLOYS OF IRON This invention relates to the separation and removal of carbon from ferroalloys, iron and alloy steels, especially those containing chromium and vanadium, hereinafter referred to as ferrous metals so that a predetermined composition of the metal can be obtained.

Many ferrous metals of commercial and industrial importance contain carbon. The physical and chemical properties of many of these metals are profoundly influenced by carbon and it is of importance to be able to separate carbon from them in order to obtain the physical and chemical properties required for specific applications in industry.

The effect of carbon is more important in the metallurgy of ferrous metals and this invention is concerned in the main with removal of carbon from such metals.

Carbon is at present almost universally removed from ferrous metals excepting ferroalloys such as ferrochromium, ferrovanadium, and the like, by treatment with cold oxygen or air in the various forms of converter well known in the steel industry or by oxygen lancing or jetting in the electric furnace. Carbon removal to low values, e.g. 0.08 percent or less in this manner by air and/or oxygen is unsuited to ferrous metals containing substantial amounts of chromium and/or vanadium, for instance more than about 7 percent of chromium and more than about 0.25 percent of vanadium, for the reason that a large proportion of the chromium and/or vanadium is oxidized by this process and passes as metal oxides into the slag.

Ferroalloys such as ferrochromium, ferrovanadium and the like which have a low carbon content of the order of 0.05 percent or less are made by reduction of the metal oxides by aluminum or silicon.

means of a plurality of alternating or direct current arcs which are transferred to the metal which is molten and superheated.

In this process at least two arcs of opposing polarity are used in conjunction with inert gases such as argon and helium are reactive gases such as carbon oxides, methane, natural gas, nitrogen, air and the like, at least one of the arcs being operated with the inert types of gas and the other arcs operated with the reactive gases. Those arcs with inert gases are referred to as heating arcs and those with reactive gases are referred to as refining or treatment arcs. Because of the complexity of maintenance on the industrial scale of a plurality of arcs of opposing polarity and of high power input, for example of power inputs of over 5000 kilowatts, and because of the vary high cost and scarcity of the inert gases .argon, helium and the like this process has not generally been applied to the removal of carbon from ferrous metals on the industrial scale.

It is the object of this invention to provide a process for the removal of carbon from ferrous metals especially those containing chromium and vanadium from levels of the order of 6 percent or higher to levels below 0.05 percent on the industrial scale. By industrial scale is meant for example, to remove carbon within these limits from a weight of molten and superheated ferrous metal of about tons at the lower limit to 300 tons or more at the upper limit in a period of the order of one hour or less.

According to this invention there is provided a method of separating carbon from ferrous metals comprising:

a. The preparation of a molten, superheated and agitated bath of ferrous metal;

b. generating a high velocity-high temperature plasma jet from a gas having an affinity to form stable volatile carbon compounds but a passivity towards the ferrous metals at such high temperatures;

c. treating the metal bath and the jet to enable stable volatile carbon compounds to be formed; and

e. enabling the carbon compounds so formed to separate from the ferrous metal.

An important further feature of this invention provides for the plasma jet to be a water vapor plasma jet which may be substantially entirely water vapor or a mixture of oxygen and nitrogen and water vapor or nitrogen and water vapor but which contains not less than 70 g./m. by weight of water vapor and such jet is hereinafter referred to as a water vapor plasma jet.

Further features of this invention provide for the ferrous metal to be poured in the molten and superheated state into a vessel which may be preheated and lined with steel-clad magnesite or chrome magnesite in such a manner as to provide electrical contacts with at least one metal wall of the vessel for the water vapor plasma jet to be substantially vertically directed and obtained from an alternating or direct current are plasma torch operating with an are which may be partially or wholly transferred to the molten and super-heated metal in the vessel. The invention also provides for the said water vapor plasma arc to be arranged to operate at a voltage suffcient to force the current through the arc plasma torch to give the desired power input to form the water vapor plasma jet at the desired high temperature and to overcome the resistance of the refractory lining of the vessel acting with its metal charge which if operated with direct current, acts wholly or partly as the anode of the assembly, so as to provide sufficient power dissipation in the inner region of the lining to cause the molten and super-heated metal bath to be thermally stirred, and if operated with direct current, to cause part of the carbon present in the ferrous metal to migrate by electrolytic action to the region of contact of the transferred arc with the molten and superheated ferrous metal, and for the metal after the water vapor plasma jet treatment, and further alloying if necessary, to be subjected where necessary to vacuum or jet degassing prior to casting. 1

Still further features of this invention provide for the ferrous metal to be maintained in a molten and superheated state in an induction heated and/or inductively stirred vessel.

It is a further feature of this invention that when alternating current is used in the arc of the water vapor plasma jet a capacitive reactance in the form of a stepwise adjustable static condenser or a rotary condenser is connected in shunt across the terminals of the are, so that a leading current of the kVAR necessary to improve the waveform and the power factor of the arc to the desired degree may be obtained.

it will be understood that the actual proportion of water vapor to admixed air and/or oxygen and/or nitrogen in'the water vapor plasma jet being used may be varied in accordance with the ferrous metal, and with the content of chromium, vanadium, phosphorus sulfur and the like elements in the ferrous metal, and that the content of water vapor may be varied at will within the limits specified. It will be understood that other gases such as carbon dioxide and the like may be added at will to the water vapor plasma jet but it is considered that these other gases will usually not offer much economic advantage. Nitrogen would normally be added to the water vapor plasma jet if nitrogen is to be a constituent of the final ferrous metal.

The invention will enable a water vapor plasma jet to have sufficient velocity to penetrate into the ferrous metal at a temperature sufficiently high to cause the hydrogen and oxygen of the water vapor and any oxygen and nitrogen present in the plasma in the region of contact of the arc plasma and metal to combine with the carbon in the metal by virtue of the negative entropy terms in the free energy equations of these reactions for separation of the carbon from the metal as volatile carbon compounds and to have a temperature sufficiently high to suppress to allow degree the reaction of the oxygen of the water vapor and any other oxygen present with the ferrous metals themselves by virtue of the positive entropy term in the free energy equations of these reactions with the said ferrous metals.

In a preferred form, the vessel used for the practical application of this invention would be in the form of a side-blown steel-making converter vessel in which the air and/or oxygen tuyeres are replaced by the water vapor plasma arc torch which is mounted in the position normally occupied by the blast tuyeres and so positioned as to direct the water vapor arc plasma jet onto the surface of the molten ferrous metal in the same way as the air blast is directed in steel-making in sideblown converter vessels. The vessel would be preheated in any series of runs and would be loaded with molten and superheated ferrous metal or the charge of ferrous metal would be melted and superheated in the converter vessel by the arc plasma. This superheated metal would be subjected to the water vapor plasma jet, and discharged on completion of carbon removal to the desired extent in the same manner as that usually adopted in side-blown steel converter practice.

As a practical example the operation of the process is envisaged to be as follows, the description of the operation being based in this example on carbon removal from a charge of ferrous metal composed of 40 percent by weight 16-18 percent chromium stainless scrap steel of A.I.S.I. 430 specification, 25 percent high carbon ferrochromium of 6.2 percent carbon 1.4 percent silicon and 58 percent chromium content, the balance being steel scrap containing 0.25 percent carbon, the vanadium content of the scrap being such that the mix contains 0.65 percent vanadium. A weight of about tons of this ferrous metal would be melted in an arc or induction furnace, superheated to approximately 100 Centigrade degrees above its melting temperature and transferred by means of a ladle crane and top-pour preheated ladle to a preheated 50 cubic foot working space capacity converter vessel as hereinbefore described which is lined with 9 inches thickness of a standard grade of steelclad magnesite bricks formed to the inside of the vessel and installed with a /4. inch to 1% inch space at the back of the clad magnesite packed by ramming with crushed 75 percent ferrosilicon or ferrosilicon-chromium in order to provide electrical contact between the clad magnesite and the vessel walls.

' The vapor plasma arc torch for this example will have an insulated water-cooled copper electrode or other suitable type of electrode and a water-cooled steel arc tube and will be designed to operate at a variable impressed voltage of up to 4.5 kv. so as to pass at maximum voltage an alternating current of approximately 650 amperes through the arc torch on to the molten ferrous metal surface in the converter vessel thereby causing a power input of up to approximately 2000 kw. into the water vapor plasma at a voltage drop of approximately 3.5 kilovolts in the arc, and a power input of approximately 350 kilowatts at a voltage drop of approximately 600 volts, from the molten metal to the converter walls through the metal-clad magnesite lining at a power factor of approximately 0.85 obtained as hereinafter described. The water vapor plasma jet would be designed to pass up to 1200 lbs. of water vapor in 40 minutes through the arc plasma stream, together with any desired mixture of additive gases Within the limits specified, the heat input into the water vapor plasma stream at maximum electrical input being approximately 8.0Xl0 B. Th.U. per hour, allowing for heat loss to the arc tube walls.

The converter vessel would be proportioned to permit when in the operating position an area of contact of the molten and superheated ferrous metal with the steel-clad magnesite brick wall lining of approximately 50 to 60 ft. depending on the converter angle and the passage of current by virtue of the transferred are or partially transferred are through the mag nesite and its steel cladding (which itself would provide a current path of cross reaction totaling approximately 200 square inches of steel which would cause thermal stirring of the metal.) Stirring of the metal could also be obtained by the mechanical action of the side-blown plasma jet and if desired could also be obtained by phase-displacement extra-low frequency induction stirring in which case the converter vessel itself would be constructed of metal having the required low curie point such as stainless steel.

More particularly in this example of the application of this invention the assembly of the water vapor plasma arc torch and the converter vessel as hereinbefore described would be arranged as illustrated diagrammatically in FIGS. 1 and 2. In

FIG. 1 the refractory-lined converter vessel 1 which is mounted on trunnions 2 is shown in the loading position with the plasma torch 3 remote from the molten ferrous metal charge 4 which has been poured in the molten and superheated condition from a top-pour ladle into the preheated converter vessel. The trunnions permit the rotation of the converter vessel through 360 and the vessel is mounted over a pit in the manner usual in operation of side-blown converters. In FIG. 2 the converter vessel is shown rotated into the operating position wherein the ferrous metal surface 4 is now near to the orifice 5 of the water vapor plasma arc tube 6 and when the plasma arc is operating the water vapor plasma flame can imp inge as a strong jet on the surface of the molten ferrous metal with a velocity sufficient to penetrate the slag layer above the metal and for some distance into the molten metal in the manner of the cold air jets of the conventional side-blown converter so that the desired reactions of the water vapor with the carbon of the ferrous metal may take place and at the same time the metal may be stirred by the mechanical action of the jet. The preferred general arrangement of the plasma are components is shown in FIG. 3 in which 7 is the water-cooled copper electrode insulated from the water vapor and gas chamber 8 at the top of the arc tube 9 which may be a plain steel tube or a water cooled steel tube. The chamber and are tube are mounted in electrical contact with the steel body of the converter and the chamber 8 is preferably cylindrical and made so that the water vapor stream or water vapor and admixed gas stream enter its sidewalls tangentially so as to cause the water vapor and gases in the arc tube to pass down the arc tube with a swirling action. The are is struck between the insulated electrode 7 and the top of the arc tube 8 by a high frequency discharge or other electrical device or by some other suitable means such as an electrically augmented combustion flame or by mechanical contacts. The are is carried down the arc tube by the water vapor stream or the water vapor and gas stream. Depending on the velocity of the water vapor stream or the water vapor and admixed gas stream, the arc continues either as a nontransferred arc by striking at varying positions on the inner surface of the arc tube, or as a partly or wholly transferred are striking wholly or in part on to the surface of the molten ferrous metal. The converter shell and therefore both the arc tube and the ferrous metal are all connected to the power source to complete the circuit. If direct current is used the insulated electrode 7 would act as the cathode of the assembly. On completion of the desired reactions the converter vessel would be discharged into a ladle in the manner of a side-blown steel converter. In order to permit free rotation of the converter body through 360 to provide for charging discharging, andcleaning, power would be transmitted to the are by means of suitable slip rings, and the water vapor or water vapor and admixed gases would be passed to the converter via a suitable labyrinth or other rotating gastight joint in the piping system. In FIG. 3 10 is the insulator which may be of a standard type of porcelain insulator electrically insulating the electrode from the converter shell, 11 is a bustle pipe admitting water vapor or water vapor and admixed gases to the chamber 8, whichis secured in a position to give the required arc length as shown in FIGS. 2 and 3 to the converter shell 1 by bolts 12. The cooling water inlet and outlet are shown as 13 and 14 respectively. The are tube 9 may be a thick-walled replaceable steel tube as shown in FIG. 3 or may be water cooled if desired. Arc tube 9 protrudes through the refractory wall of the converter to the orifice 5 of FIGS. 1,2and 3.

In this embodiment of the invention there would be connected across the insulated electrode 3 of FIG. 1 a rotary condenser of maximum capacity of 2,000 K VAR at the operating voltage of the arc so that an infinitely variable leading reactive current can be injected into the circuit for correction of the form factor of the potential wave and the power factor of the circuit.

letting would continue under these conditions until the desired carbon content of the ferrous metal is attained which should be within 60 minutes of elapsed jetting time. Interruptions of jetting may be necessary for temperature control, ferrous metal or alloy additions or slag removal. The ferrous metal would then be transferred by ladle crane after discharge of the converter vessel, for casting or if desired for vacuum degassing in, for example, an induction stirred ladle degassing plant or the like, the casting after degassing.

The composition of the metal, except for its vanadium content, would conform to the A.l.S.l. 43 specification, would be virtually unaltered except for minor losses of constituent metals in the ferrous metal and, except that the carbon content would be reduced to less than 300 parts per million, it could reasonably be expected that some or all of the silicon content of the metal, if silicon is present, and that, under a suitable basic slag, some or the greater part of the phosphorus and sulfur contents, if these elements are present in significant amounts, would also be removed.

A second practical example, the operation of the process, is envisaged to be as follows, the description of the operation being based in this example on carbon removal from high carbon ferrochromium containing 6.5 percent carbon, 58 percent of chromium and 2.3 percent of silicon.

A weight of about l0 tons of this ferrous metal would be melted in an are or induction furnace, super heated'to approximately 100 Centigrade degrees above its melting temperature and transferred by means of a ladle and top pour preheated ladle to a preheated 50 cubic foot working space capacity converter vessel as hereinbefore described which is lined with 9 inches thickness of a standard grade of steel clad magnetic bricks formed to the inside of the vessel and installed with a V4 inch to 1% inch space at the back of the clad magnesite packed by ramming with crushed 75 percent ferrosilicon or ferrosilicon-chromium in order to provide electrical contact between the clad magnesite and the ladle walls.

The vapor plasma jet torch for this example will have a water-cooled copper cathode or other suitable cathode and a steel arc tube which may be water cooled if desired and will be designed to operate at an impressed voltage of 6.6 kv. so to pass a current of approximately 700 amperes through the arc torch on to the molten ferrochromium surface in the cylindrical ladle thereby causing a power input of approximately 3,500 kw. into the water vapor plasma at a voltage drop of approximately 6.0 kilovolts in the arc tube, and a power input of approximately 350 kilowatts at a voltage drop of approximately 600 volts, from the molten metal to the ladle walls through the metal clad magnesite lining at a power factor of about 0.85 obtained as hereinbefore described. The water vapor plasma jet would be designed to pass up to 4,000 lbs. of water vapor and if desired up to 2,000 lbs. of admixed air or nitrogen with correspondingly less water vapor in 40 minutes through the arc plasma stream, the heat input into this vapor stream being approximately 13x10" B.t.u. per hour.

Jetting would continue under these conditions until the desired carbon content of the ferrochromium was attained which should be within 60 minutes of elapsed jetting time. Interruptions of jetting may be necessary for temperature control, ferrous metal or alloy additions or slag removal. The ferrous metal would then be transferred by ladle crane after removal of the plasma torch, for casting or if desired for vacuum degassing in, for example, an induction stirred ladle degassing plant or the like, and casting after degassing.

The composition of the metal would again be virtually with the same exceptions given in the first example.

It is to be noted and would be apparent to those versed in the art that the plasma torch could also in an alternative arrangement be suspended by a suitable hoist suspension so that is could be lowered through a conical refractory lined splash guard placed on top of a cylindrical ladle through an aperture wide enough to pass the torch and to allow the volatile carbon components separated from the ferrochromium bath to escape to atmosphere. The suspension of the plasma arc and the free-board of the ladle itself would be arranged to allow the torch to operate at various heights above the metal during the course of treatment, for example, from say mms. to 700 mms. above the metal surface depending upon operating conditions and the slag volume generated during treatment.

The assembly of plasma torch and cylindrical ladle would be electrically connected so that if direct current is used the vessel and therefore its molten metal contents would be the anode and the plasma torch would operate with a watercooled copper cathode or an equivalent cathode.

The cylindrical vessel would be proportioned to permit an area of contact of the molten and superheated ferrous metal with the steel-clad magnesite brick wall lining for example with a 10 ton charge of ferrous metal of approximately 50 square feet and the passage of a current of approximately 600 to 800 amperes through the hot magnesite and steel cladding, itself providing a current path of cross section totaling approximately 200 square inches of steel, would effectively cause thermal stirring of the metal. Alternatively or additionally stirring could be obtained by the other means set out above.

It is also to be noted that the metal being treated as hereinbefore described by alternating current arc, transferred direct current are or nontransferred direct current arc or a plurality of such arcs, would normally be covered by a suitable basic slag layer formed partly from elements oxidized out of the metal combining with material from the lining and partly by slag-forming materials which have been purposely added to the required depth in order to protect the metal from interaction with the cooler atmosphere above the metal away from the water vapor plasma jet, such slag layer being of a thickness and fluidity permitting penetration of the water vapor plasma jet through the slag to the metal or made so by adjustment of composition and quantity of slag in the converter. It would also be apparent to those versed in the art that a protective slag such as that hereinbefore described need not be molten and of high fluidity but could be scoriaceous although it would be advantage to have a molten slag of high fluidity.

It will further be apparent to those versed in the art that the water vapor required for the plasma torch as hereinbefore described may be generated in a separate boiler and superheater and then passed into the arc via chamber 8 of FIG. 3 as hereinbefore described or may be generated in a suitably placed oxy-fuel or air-fuel burner into which the requisite water (if any) is fed for vaporizing and superheating. The combustion gases from the burner would then pass into the arc tube along with any added water vapor together with the water vapor generated in the combustion and, if desired, the fuel burner may be so positioned that its combustion region and hot combustion gases may be electrically augmented or power assisted to a desired degree by the arc plasma. This power assistance of combustion gases has already been proposed for a different application of a plasma torch and descriptions of the proposal are available in the literature in this art.

The invention enables carbon to be removed from ferrous metals expeditiously and efficiently while utilizing plant which may be easily controlled and which is not excessively expensive to install.

What I claim as new and desire to secure by Letters Patent is:

1. A method of separating carbon from ferrous metals comprising:

a. The preparation of a molten, superheated and agitated bath of ferrous metal;

b. generating a high velocity-high temperature water vapor plasma jet from a gas having an affinity to form stable volatile carbon compounds but a passivity towards the ferrous metals at such high temperature;

0. penetrating the metal bath with the jet to enable stable volatile carbon compounds to be formed; and

d. enabling the carbon compounds so formed to separate from the ferrous metal.

2. A method as claimed in claim 1 in which the plasma jet is a mixture of water vapor and nitrogen containing not less than 70 g./m. of water vapor.

3. A method as claimed in claim 1 in which the plasma jet is a mixture of oxygen, nitrogen and water vapor containing not less than 70 g/m. of water vapor.

4. A method as claimed in claim 1 in which the water vapor for the plasma jet is generated in a separate boiler and superheater.

5. A method as claimed in claim 1 in which the material for the plasma jet is obtained from an air-fuel or oxy-fuel burner.

6. A method as claimed in claim 5 in which water is fed to the burner for vaporizing and superheating.

7. A method as claimed in claim 5 in which the hot combustion gases are power assisted by the arc plasma.

8. A method as claimed in claim 1 in which the metal bath is held in a vessel lined with steel-clad refractory material to provide electrical contact between the bath and wall of the vessel.

9. A method as claimed in claim 8 in which the refractory material is magnesite or chrome magnesite.

10. A method as claimed in claim 8, in which the bath is held in a side-blown steel converter vessel.

11. A method as claimed in claim 8, in which-the vessel is preheated prior to the introduction of the molten metal bath.

12. A method as claimed in claim 1 in which the plasma jet is obtained from a plasma torch powered by either an alternating or direct current electrical supply.

13. A method as claimed in claim 2 in which the arc ferred to the bath of molten metal.

14. A method as claimed in claim 13 in which the plasma is transmetal bath forming the anode of the assembly.

16. A method as claimed in claim 5 in which suflicient power dissipation is effected in the inner region of the lining to cause the molten and superheated metal bath to be thermally stirred.

17. A method as claimed in claim 5 in which part of the carbon present in the molten metal bath is caused to migrate by electrolytic action to the region of contact of the arc with the molten and superheated metal.

18. A method as claimed in claim 1 in which the molten metal is subjected to a degassing process after treatment with the plasma jet.

19. A method as claimed in claim [in which the plasma torch is powered by an alternating current supply and a capacitive reactance connected in shunt across the terminal of the arc to obtain a leading current near to the arc terminals.

20. A method as claimed in claim 19 in which the reactance is obtained by means of a stepwise adjustable static condenser or a rotary condenser.

* I k i 

2. A method as claimed in claim 1 in which the plasma jet is a mixture of water vapor and nitrogen containing not less than 70 g./m.3 of water vapor.
 3. A method as claimed in claim 1 in which the plasma jet is a mixture of oxygen, nitrogen and water vapor containing not less than 70 g/m.3 of water vapor.
 4. A method as claimed in claim 1 in which the water vapor for the plasma jet is generated in a separate boiler and superheater.
 5. A method as claimed in claim 1 in which the material for the plasma jet is obtained from an air-fuel or oxy-fuel burner.
 6. A method as claimed in claim 5 in which water is fed to the burner for vaporizing and superheating.
 7. A method as claimed in claim 5 in which the hot combustion gases are power assisted by the arc plasma.
 8. A method as claimed in claim 1 in which the metal bath is held in a vessel lined with steel-clad refractory material to provide electrical contact between the bath and wall of the vessel.
 9. A method as claimed in claim 8 in which the refractory material is magnesite or chrome magnesite.
 10. A method as claimed in claim 8, in which the bath is held in a side-blown steel converter vessel.
 11. A method as claimed iN claim 8, in which the vessel is preheated prior to the introduction of the molten metal bath.
 12. A method as claimed in claim 1 in which the plasma jet is obtained from a plasma torch powered by either an alternating or direct current electrical supply.
 13. A method as claimed in claim 2 in which the arc is transferred to the bath of molten metal.
 14. A method as claimed in claim 13 in which the plasma torch is arranged to operate at a voltage sufficient to force the current through the arc plasma torch to give the desired power input to form the plasma jet at the desired high temperature and to overcome the resistance of the refractory lining of the vessel acting with its metal charge.
 15. A method as claimed in claim 4 in which the torch is powered by a direct current supply with the vessel and molten metal bath forming the anode of the assembly.
 16. A method as claimed in claim 5 in which sufficient power dissipation is effected in the inner region of the lining to cause the molten and superheated metal bath to be thermally stirred.
 17. A method as claimed in claim 5 in which part of the carbon present in the molten metal bath is caused to migrate by electrolytic action to the region of contact of the arc with the molten and superheated metal.
 18. A method as claimed in claim 1 in which the molten metal is subjected to a degassing process after treatment with the plasma jet.
 19. A method as claimed in claim 1 in which the plasma torch is powered by an alternating current supply and a capacitive reactance connected in shunt across the terminal of the arc to obtain a leading current near to the arc terminals.
 20. A method as claimed in claim 19 in which the reactance is obtained by means of a stepwise adjustable static condenser or a rotary condenser. 